Since Darth Bandon is not a choice and General Grievous is neither Sith nor Jedi, then my choice would have to be Atris, simply by elimination.
Nahilus- He destroys entire planets. His character was simply underdeveloped. It has been theorized that he may be returning in Kotor 3 as a major adversary. He is nowhere near weak.
Count Dooku- Went toe to toe with three of the toughest jedi of his time and managed to hold his own. Sure, he fled from Yoda, but not before putting up a good fight. Plus, he managed to best both Anakin and Obi-Wan. He was only defeated after being weakened by Obi-Wan.
Darth Maul- No force powers to speak of. He was, however, the closest thing to a "True Sith" that either the movies or the games has had. Plus, he was amazing with a lightsaber.
Malak- Dark Lord of the Sith. Please. How is he weak?
Revan- Dark Lord of the Sith. Hero of the republic. Kotor 1 antagonist. Either bested Malak and Bandon or Malak, Bandon, and all of his party depending on how you played it.
Mace Windu- Regarded by many as the best saber fighter of his time. Was no slouch with the force, either. Bested Palpatine and was only defeated when Anakin betrayed him.
Sion- He is actually the most powerfull, simply because he defied the laws of life and death. He was only defeated when The Exile convinced him to give up his life force.
Luke- If you only watched the movies, then I could see why you could make a case for him being the weakest. He often struggled against non-force users and failed to best the only 2 force users he faced. However, the books and the Jedi Knights series show us that, in time, Luke becomes the most powerful force user of his time.